Director Layering triggers domain_timeout, consequently no backend is returned

[nigoroll]

When layering a dynamic director under another director, the last use is not updated and, when the domain timeout expires, the director stops returning backends (FetchError Director ONTOP returned no backend with ONTOP being the director having configured a dynamic domain as a backend). I guess this use case makes the whole concept of a domain timeout useless, but I'll need to ponder this for a bit.

[nigoroll]

Outcome of pondering:

• domain_usage_timeout should continue to exist: It it the timeout for how long the vmod object holds a reference to a director for a domain.
• Irrespective of the timeout, the domain director should continue to function (perform DNS lookups, return backends) for as long as a reference exists.

In d1f5b141a85ede85cfcde987f665ce4559836c30 and other commits, I already started to move control away from the object to the domain director, and this renovation needs to be finished. Also control over the thread needs to be coupled to the domain director life cycle, considering also the temperature transitions.